The Nelson Multi is a bright emitter triode with three straight wire tungsten filaments and a switching arrangement on the base. The idea was to both increase the life of the valve and to provide a double functionality.
One filament could be selected and thus provide three times the normal life for a low power valve operating as a detector or AF amplifier. The top operating characteristics table then applies. Additionally a pair of filament could be switched into circuit at once. This provided extra emission and allowed the valve to operate as a power valve.
The instructions with the valve state:-
The valve contains three filaments which can be brought into use either one or two at a time, thereby lengthening the life of the valve and adapting it for power amplification.
The switch consists of two arms pivoted scissors-like on one of the cap legs. When it is desired to use one of the filaments the arms should be brought together with the tips in contact with one of the three studs in the valve base.
When two filaments are required the arms should be opened in opposite directions so that the tips are in contact with any two of the studs.
The gettering obscures all internal details. The paper label showing more details.
More of the paper band. A filament rating of 3-4 Volts is present because bright emitters would be operated with a rheostat in series with the filament and the current limited to just the amount required for adequate emission/reception.
The The final part of the paper band.
The paper sticker of the second type of BBC stamp. This dates this exhibit to between September 1924 and 1927.
The two filament switches on the base pin. The base wires are soldered to the simple brass split pins.
The filament wires are copper and just sit within the slot of the machine screw in the base.
The balloon envelope is 38 mm in diameter, and excluding the B4 base pins is 93 mm tall.
Reference: 1003. Type Multi was first introduced in 1924.